Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a mask utilized in the photolithography process may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material. In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via a projection system, one at a time.
In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus-commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. In general, the projection system will have a magnification factor M (generally <1). The speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be found, for example, in U.S. Pat. No. 6,046,792, which is incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, the mask pattern is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g., an IC.
Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer).
These devices are then separated from one another by a technique such as dicing or sawing. Subsequently, the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”. However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
Of course, one of the goals in integrated circuit fabrication is to faithfully reproduce the original design on the wafer (via the mask). As the demand to image smaller and smaller features in the semiconductor manufacturing process has continued unabated, the limitations of optical lithography that were once accepted have been exceeded repeatedly. However, printing smaller features on a chip requires improving the resolution of the optical system by increasing the numerical aperture and/or decreasing the exposure wavelength. For this reason, the wavelength of microlithographic tools has progressed from historic g-line (436 nm) and I-line (365 nm) to the recent KrF (248 nm) and ArF (193 nm) laser generations, with 157 nm slated to follow. Beyond 157 nm, the use of transmissive optical materials becomes impractical and the next generation of tools will require reflective optics and an extreme UV (EUV) light source at 13.4 nm.
The optical material requirements for these lithographies are highly dependent on whether the material is used for laser optics, illumination systems, photomasks, projection optics, or inspection tools. Common to all is a need for outstanding transmission (or reflection, in the case of EUV). Depending on the type of optic, this requirement can stem simply from the need for high throughput; however, in the case of projection optics, contamination on a surface of a lens may also cause errors in the image projected onto the substrate, which in turn may result in defective devices or degraded device performance. For example, contamination that attaches to a lens surface may cause unwanted scattering of the illumination source. In some cases, the scattering causes a feature (closed space) to form in an image where an open space should be or vice versa In some cases, the projection characteristics of the lens are significantly degraded. For example, a lens with significant degradation may produce features with a critical dimension greater than the design specifications.
Further, a significant amount of time may be spent in removing contamination from surfaces of the lithographic lens. Even with routine cleaning over a period of time, a significant amount of contamination may build up on a surface of the lithographic lens requiring the lens to be replaced.
As such, there is a need for preventing or inhibiting contamination that attaches to a surface of the lithographic lens from adversely affecting an image projected by the lithographic lens.